One thing that was clear watching Guardians of the Galaxy was that origins of many of the characters had been altered from their comic origins. In my eyes the reason for the refresh was because overtime these characters evolved to the point where their origins made little sense. At other times the origin may not fit, or be too complicated in film. Especially a film that focuses on a team and has limited amount of time to focus on each character. This is true of Star-Lord, Drax and Groot whose really origins are complicated and could take too long.
Other origins may have changed because tonally they did not match the film. This is true of Rocket Raccoon whose origin was kept similar but altered in tone. In essence Rocket's movie origin was made darker than his is original sillier origin made by Bill Mantlo in Incredible Hulk #271.

Rocket's Comic Origin:

In the original version by Bill Mantlo, Rocket Raccoon is from planet called Half World. Many years ago human like aliens shipped off their mentally ill to the planet Half World. They programmed robots to be their caretakers and sent animals to keep them company. The planet is blocked off from the rest of the universe by a barrier. Things go on like this for many years, apparently the mentally ill population reproduces new mentally ill offspring so that facility continues to run ad infinitum. An event occurs where the robots become sentient and they become miserable simply watching after the mentally ill. They decide to leave the planet and begin to construct a massive space ship. They do not wish the mentally people ill however, so they begin altering the animals on the planet to fulfill various purposes. Rocket Raccoon was constructed to act as a marshal security officers. The robots then blast off, taking half of the planet with them. The remaining portion of the world is thus called Half World.

Rocket lives as the marshal of the world and deals with local villains and politics for a long time. He has a best friend Wal Russ and a otter girlfriend. However, after receiving visitors from the off the planet he sets off to leave the planet and see what else is out there.

Many years later when Rocket began to gain popularity in Guardians of the Galaxy it was suggested that portions of this old origins were falsely imprinted memories. Thus the original origin has been partially retconned in comic form as well. It is unclear what of the original story is considered real and what was part of imprinted memories.

Rocket's MCU Origin:
The animated series has revealed an origin that is likely close to what the MCU origin will be.
In this origin, Rocket is a experimental weapon / soldier modified at a biological weapons facility called Half World. Rocket is a small but scrappy raccoon who has been filled with mechanical and tactical information. After his creation he chaffs at the attempts of being controlled by machines running facility. He allies himself with a tree creature at the facility named Groot and the two escape. Upon his escape he takes the name of Rocket for himself after creating and using a rocket to escape to escape.